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Dead weakrefs to a given object happen to be equal if they don't have a callback, but unequal if they do. However, they are always equal when alive:
>>> class O: pass
... 
>>> o = O()
>>> def cb(_): pass
... 
>>> q = weakref.ref(o)
>>> r = weakref.ref(o)
>>> s = weakref.ref(o, cb)
>>> t = weakref.ref(o, cb)
>>> q == r
True
>>> s == t
True
>>> del o
>>> q() is None
True
>>> q == r
True
>>> s == t
False
This may be related to the following optimization (?):
>>> q is r
True
>>> s is t
False